Most tank enclosures for pets simply have four walls and a bottom. The pets are typically prevented from escaping by a removable plastic cover. Many people prefer to have more than one type of environment in their tanks to house different types of animals in the same tank. As a result, many people pile soil or gravel on part of the tank bottom for a land or terrestrial environment, then put water in the tank to have an aquatic environment as well. This allows the keeping of land animals, amphibious animals, and aquatic animals all in the same tank. However, piling gravel or soil on the bottom of the tank greatly reduces the amount of space available for the aquatic environment, which is highly undesirable. In addition, soil and minerals can foul the water and clog filters and pumps used for the aquatic environment.
One solution to this problem is to use a removable cover as a terrestrial environment, place additional walls above the removable cover, and place a second removable cover on the additional walls to prevent escape of the animals. In effect, there are two tanks: an aquarium on the bottom and a terrarium on top, the terrarium having a hole in its bottom so animals that want to travel between the tanks can do so. While this solves the problem to a point, maintenance of the aquarium can be done only by removing the top walls. To ease maintenance of the aquarium and the terrarium, doors are placed in one of the second set of walls; but the doors provide an escape route for animals in the terrestrial environment when the doors are open.